1. Field of the Invention
The invention relates to electrolytic apparatus for generating and purifying hydrogen. More particularly, the apparatus of the invention relates to the purification of impure hydrogen by permeation through the walls of a plurality of palladium-silver tubes.
2. Description of the Prior Art
As shown by U.S. Pat. No. 1,174,631 to Walter O. Snelling, it has long been known that a palladium wall will pass hydrogen in ionic form and will allow the hydrogen to recombine into molecular form so as to purify gases which include hydrogen as a constituent to obtain pure hydrogen. The permeability of hydrogen through palladium walls is dependent upon the thickness of the wall, the pressure differential on the sides of the wall, the temperature of the wall and the material from which the wall is made. Within operating ranges, the permeability is directly proportional to the pressure differential. The permeation rate increases exponentially with the increase in temperature. U.S. Pat. No. 2,773,561 to James B. Hunter teaches that a silver-palladium composition containing from 10 to 50% silver provides increased permeability over pure palladium. A number of hydrogen generators which use electrolytic action and palladium tube purifiers are on the market.
Early prior art generators using palladium tube purifiers used the palladium tube as the cathode which allows all of the positive ions in solution to migrate to the cathode to be reduced. Since only hydrogen ions pass through the palladium, all other ions tend to form surface alloys on the palladium tube and therefore tend to rapidly reduce hydrogen permeability. This problem is particularly critical because of the expense of such tubing.
U.S. Pat. No. 3,761,382 represented a breakthrough in the art of hydrogen generation by teaching a unitary, portable apparatus comprising a generation unit, a purifier unit, a water supply and a pump unit. The purifier of U.S. Pat. No. 3,761,382 is unique in the prior art since it is separated from the generator unit. Thus, the caustic electrolytic solution is not exposed to the purifier which means that surface alloys will not readily form on the silver-palladium tube. The device of the present invention specifically improves on the purifier described in U.S. Pat. No. 3,761,382. The invention is also useful as an inexpensive means for purifying bottled hydrogen gas as well as other sources of hydrogen.
Impure hydrogen gas is introduced into a pressurized chamber in the purifier of U.S. Pat. No. 3,761,382. The chamber contains a palladium-silver purifier tube arranged in helical turns. A pressure differential exists between the chamber and the inside of the palladium-silver tube thereby causing pure hydrogen to pass from the chamber to the inside of the tube and eventually to the pure hydrogen outlet. The temperature of the chamber and tube is maintained at the desired operating temperature by an electrical heating unit.
The major drawback of the patent purifier is that the input pressure of impure hydrogen gas is greatly limited because it is difficult to safely maintain a high pressure in the chamber. The patented purifier has generally been operated with a maximum chamber pressure of 400 psig. The chamber must be leak-proof because of the explosive nature of hydrogen. Also, the chamber must prove to be explosion-proof by meeting the requirements of a Class 1, Group B, enclosure as set forth in the National Electrical Code.
The impure hydrogen introduced into a purifier from bottle sources will occasionally contain air which has leaked into the line during a change of bottles. This air will react with the hydrogen and can cause an explosion with a pure H.sub.2 O product. A drawback of the patent purifier is that a relatively large volume of impure hydrogen is contained within the chamber during operation. In the event of a leak of air into the chamber, this volume of hydrogen could react with the oxygen in the air and cause a dangerous explosion. The present invention greatly reduces the amount of impure hydrogen which is confined during operation; therefore, in the event of such an explosion, the explosion will be much less severe. In fact, the explosion will be contained within the jacket tube without any possibility of external damage.
It has been found that the prior art purifiers having a single palladium-silver tube are severely limited in capacity because there is a maximum flow rate of gas that may pass through the tube without creating turbulent flow. That is, with a single palladium-silver tube design operating at a given temperature and pressure differential, the capacity of the purifier can be increased only by increasing the length of the tube or by increasing the I.D. of the tube. However, the length may be increased only up to a certain limit because of the mentioned maximum flow rate. An increase in the I.D. necessitates an increase in the wall thickness of the tube in order to maintain the same burst characteristics, thus slowing down the permeation of pure hydrogen through the wall. Therefore, it becomes an object of the present invention to overcome these disadvantages by providing a workable hydrogen purifier having multiple palladium-silver tubes. These tubes are located within a single pressurized jacket tube so as to provide a compact and safe unit capable of easy assembly.